The process of TW-electroerosion generally makes use of a continuous wire electrode composed of, say, brass or copper, or brass- or copper- coated steel, and having a thickness or diameter ranging between 0.05 and 0.5 mm. The term "wire electrode" or "electrode wire" has been used in the art and hence is used throughout herein, to refer to a thin, continuous elongate electrode element generally and to include not only a conductive wire which may be circular, triangular, square, rectangular or polygonal in cross section but a like continuous element in the form of a conductive tape or ribbon. The electrode wire is axially transported continuously from a wire supply to a wire takeup through a cutting region in which a workpiece is disposed. The cutting region is commonly defined between a pair of guide members which support and hold the wire while traveling through the workpiece. Wire traction and braking means allow the continuous wire to be tightly stretched and kept taut under a given tension and to be axially driven between the cutting guide members while traversing the workpiece, thus presenting a continuously renewed electrode surface juxtaposed in an electroerosive cutting relationship with the workpiece across a narrow gap or cutting zone. The cutting zone is flushed with a cutting liquid medium, e.g. water, and is electrically energized with a high-current-density electrical machining current which is passed between the electrode wire and the workpiece to erode the latter or erosively remove material therefrom.
The cutting process may be performed in any of various electroerosive machining modes. In electrical discharge machining (EDM), the cutting liquid is a dielectric liquid, e.g. deionized water, and the machining electric supplied in the form of a succession of electrical pulses. In electrochemical machining (ECM), the cutting medium is a liquid electrolyte, e.g. an aqueous electrolytic solution, and the machining current is a high-amperage continuous or pulsed current. In electrochemical-discharge machining (ECDM), the liquid medium has both electrolytic and dielectric nature and the machining current is preferably applied in the form of pulses which facilitate the production of electric discharges through the conductive liquid medium.
The workpiece may be disposed in a bath of the cutting liquid medium to immerse the cutting region therein. More typically, however, the cutting zone is disposed in the air or ambient environment. Advantageously, one or two nozzles of the conventional design disposed at one or both sides of the workpiece have been employed to deliver the cutting liquid medium to the cutting region disposed in the air or immersed in the liquid medium. The cutting liquid medium is conveniently water as mentioned, which is deionized or ionized to a varying extent to serve as a desired electroerosive cutting medium.
To advance electroerosive material removal in the workpiece, the latter is typically displaced relative to the traveling wire and transverse thereto. This allows the traveling wire to advance translationally in the workpiece and consequently a narrow cutting slot to be progressively formed behind the advancing wire, the slot having a width substantially greater than the thickness of the wire. The continuous relative displacement along a precision-programmed path results in the formation of a desired contour corresponding thereto and defined by this cutting slot in the workpiece.
Higher cutting speed in the process described is ever an increasing demand in the industry and should be achieved with due precision. The cutting speed, typically expressed in mm.sup.2 /min, is defined by the product of the workpiece thickness and the length of cut achieved per unit time along a given course and hence is, for a given workpiece thickness, dependent upon the rate to which translational advance of the wire electrode can be increased. The rate of advance is in turn limited by the rate of actual material removal dependent on the one hand upon the preset cutting parameters that govern, inter alia, the cutting accuracy and on the other hand upon the conditions in the cutting zone which may instantaneously vary. If the rate of advance happens to exceed the rate of actual material removal, the fine wire may break. The goal of higher cutting speed is, therefore, dependent on the extent to which optimum conditions in the cutting zone may be established and may be maintained stably in the face of instantaneous changes. Among other factors which govern these conditions, it has been recognized that adequate flushing is of particular importance.
In the interest of increasing the cutting speed in the TW-electroerosion process, it is thus necessary that the cutting zone be flushed with the cutting liquid in a sufficient volume and at a sufficiently high flow rate, yet uniformly along its entire length, i.e. across the thickness of the workpiece, to allow the erosive action to continue with stability, the cutting chips and other erosive products to be carried away promptly and the wire subject to erosive heating to be cooled with greater effectiveness. Accordingly, the art has seen various improvements in the structures of fluid-delivery nozzles and the manner of supplying the liquid medium in the cutting zone. It has been observed, however, that they are much less than ideal. Some of them have left much to be desired from practical standpoints and the others have been found only satisfactory to substantially increase the cutting speed where the workpiece is relatively thin. The greater the workpiece thickness, the more difficult it is to achieve the same cutting speed as may be attainable with thinner workpieces.